When you creating new DataSource in WLS, it allows you to test the connectivity. However after you have created it, the test button is kinda hidden in a non-obvious way. This is how I get to it and test an existing DataSource that have targeted a server.
1. Login to Admin Console (eg: http://localhost:7100/console)
2. On the Domain Structure left panel, click:
<my_domain_name> (eg: DefaultDomain) > Services > Data Sources
3. On the right, Click on the DataSource name you want to test.
4. Click the "Monitoring" tab > "Testing" tab.
5. Select the server the DataSource has targeted to and then press "Test Data Source" button.